It is conventional to measure time by using a ring oscillator, in which plural delay elements for delaying a pulse signal are connected in a ring form, and encoding the number of stages of the delay elements, through which the pulse signal passes in a period of measurement (refer to U.S. Pat. No. 5,818,797 corresponding to JP H10-54887 A, for example).
According to the conventional device, a variation in a measured time value increases as a period of measurement increases. This variation is considered to arise, because the delay time of individual delay element forming the ring oscillator varies with a variation of a power supply voltage, thermal noise and the like and this variation of the delay time accumulates in proportion to the number of the delay elements, through which the pulse signal passed.
In an application for detecting precisely a variation in a frequency, which has a sufficiently long period longer than milliseconds (ms) relative to the delay time of nanoseconds (ns) of the delay element, the error in the measured time value increases to be large relative to the variation in the frequency to be measured. This variation thus makes it impossible to attain the time measurement precisely.
In an actual experiment, a period of a signal, which is set to a predetermined frequency (oscillation period), was measured 2,000 times. This measurement was performed 2,000 times for each frequency, while changing the predetermined frequency. The measurement exhibited the following results. That is, as shown in FIG. 15A, a period (oscillation period) OP of a measurement signal, which is a signal to be measured, and an average of measured time values (frequency control data) were in a proportional relation. However, as shown in FIG. 15B and FIG. 15C, a width of variation (difference) PP between a maximum value and a minimum value of the measured time values and a dispersion σ, which indicates an average level of variation of the measured time values, increased as the measurement period became long (as a result, as the measured time value increased). It was confirmed specifically that the width of variation PP reached about 0.02% (200 ns) and the dispersion σ reached about 0.002% (20 ns) when the measurement period was about 1 ms (near a right top in each figure).